(a) Field of the Invention
This invention relates to a radiation-curable coating composition, which undergoes curing upon exposure to radiation to provide a rubber-like cured material having excellent electrical characteristics and superb elongation and elasticity and capable of absorbing vibrations and impacts, and also to a coating composed of the cured material.
(b) Description of the Prior Art
A variety of silicone resins have heretofore been used as insulating materials for electrical devices, apparatus and equipment and the like and as coating, covering or cladding materials for absorbing vibrations and impacts. A silicone resin has a low viscosity although its curable component or components amount nearly 100%. Its cured material is a rubber-like elastomer having excellent electrical characteristics and is also outstanding in heat resistance, freeze resistance, weatherability, adhesion properties and so on. These silicone resins are however accompanied by such drawbacks that they are not only expensive but also time-consuming for their curing. Their curing reactions are widely varied in types. One-pack type silicone resins release acetic acid, oximes and/or alcohols, whereby to induce corrosion on metals or to act as causes for the formation of pin holes in cured films. On the other hand, there are also some silicone resins which are curable by addition reactions. These silicone resins are however of the two-pack type and are thus inconvenient for applications.
On the other hand, there have also been known resins which are curable by radiation such as ultraviolet ray, electron beam, etc. Each of these resins generally contains its curable component or components to a total amount of almost 100%. They have low viscosities, are of the one-pack type, and feature high curing speeds. However, cured materials obtained from conventional radiation-curable resins are usually hard and lack elongation, and hence do not have properties as so-called rubber-like elastomers. Some butadiene-acrylate base resins have properties as rubber-like elastomers, but their cured materials have poor weatherability and even when left over in the air, lose their rubber-like elasticity little by little and eventually result in hard materials. Furthermore, such conventional radiation-curable resins are accompanied by another drawback that they undergo significant shrinkage upon their curing and when used as coatings or coverings, they exhibit poor adhesion properties. For these reasons, it was not possible to use conventional radiation-curable resins in place of such coating or covering silicone resins as described above.
Reflecting the sharp increase in the production of glass-base optical fibers in recent years, there is a strong demand, especially, for an excellent cladding material which can assure good productivity not attainable by any conventional silicone resins. Namely, a jacketed fiber useful for the production of a glass-base optical fiber is produced by covering a bare glass fiber of about 100 .mu.m in diameter with a primary cladding of about 150 .mu.m thick and then with a secondary cladding of about 250 .mu.m thick. It is essential for the primary cladding to have excellent tensile elongation and also rubber-like elasticity, namely, to have a low Young's modulus in tension and to retain such properties at low temperatures. However, conventional coatings cured by radiation were not able to fully satisfy these requirements.